The present invention generally relates to providing a uniform distribution of airflow to a heat exchanger and, more specifically, to apparatus and methods to improve the distribution of a high velocity (i.e., greater than or equal to about 250 ft/sec) sub-freezing (i.e., less than or equal to the freezing point of water, less than or equal to about 32° F.) airflow provided to an inlet face of a heat exchanger.
In many applications, environmental control systems that provide cooling to various heat loads, may operate utilizing expanding fluids flowing from an outlet of a turbine. Such airflows generally have high velocities of greater than or equal to about 250 ft/sec., and may be at sub-freezing temperatures less than about 32° F., typically as cold as about −30° F. Such airflows may not be evenly distributed upon entering a heat exchanger due to the small area of the turbine exhaust compared to the flow area of the heat exchanger inlet face. For example, such airflows may contain ice or snow created through the expansion cooling of air through the turbine, which can accumulate on, and may block portions of a heat exchanger inlet face that is immediately downstream. The airflow may also be stratified (e.g., non uniformly distributed), thus being preferentially directed to a portion of the inlet face of the heat exchanger.
Accordingly, non-uniform distribution of a high velocity, sub-freezing fluid flow may prevent the heat exchanger from operating in a most efficient manner. For example, the inlet face of the heat exchanger may become blocked by snow or ice formed as the compressed fluid (e.g., air) flows through the sudden expansion at the heat exchanger inlet pan to the inlet face of the heat exchanger. This blockage may require various components of the system to be larger than would be required without the blocking of the heat exchanger face, and/or may require anti-icing measures to be deployed, all of which may reduce the system effectiveness. This problem may exist in fluid conditioning apparatus and systems utilized in, for example, aircraft refrigeration systems such as those that provide environmental controls including cooling to various liquid and other heat loads generated by various components and systems in the aircraft.
In an effort to address the above problems associated with non-uniform distribution of an expansion cooled fluid (e.g., an air flow) to a heat exchanger, such as those that may result from the blockage of the face of a heat exchanger located downstream of a turbine by ice or snow created through the expansion cooling of air through the turbine, systems directed to removal of water from the airflow prior to expansion through the turbine have been used. In one design shown in U.S. Pat. No. 4,246,963, a condenser and water collecting means may be employed prior to the airflow entering the turbine to remove entrained liquid water from the airflow. Unfortunately, a water condenser may add an additional heat load onto the cooling system, and the condenser and water collection means may add additional weight and complexity to an aircraft or other environment in which this system may be located. An anti-ice bypass of bleed air may also be mixed with the cooler expanding air flowing from the turbine outlet to maintain the expanding air above freezing before entering the heat exchanger. However, this approach may result in an energy drain on the overall system which may reduce system performance. Also, neither removal of water and/or the use of bypass air directly address stratification of the expanding airflow that may occur as it contacts the inlet face of the heat exchanger.
Another approach directed to providing uniform distribution of a high velocity, sub-freezing airflow emanating from a turbine to the inlet face of a heat exchanger includes minimizing flow velocity stratification. In one design shown in U.S. Pat. Nos. 5,025,642 and 5,214,935, a back-pressure plate is utilized after the heat exchanger to block a portion of the outlet of the heat exchanger, thus causing the airflow to be more uniformly distributed through the heat exchanger. This approach may be used in conjunction with removal of the water vapor in the airflow prior to expansion of the airflow in the turbine. However, the backpressure created from this approach may adversely affect the efficiency of the system by increasing pressure drop, and the plate and other apparatus may add weight and complexity to the aircraft or other vehicle in which the system may be operating.
Another approach directed to providing uniform distribution of a high velocity, sub-freezing airflow to the face of a heat exchanger includes the use of hollow tubes for header bars disposed directly on the face of, and in physical contact with, the heat exchanger. These tubes may be maintained above freezing (i.e., above about 32° F.) and thus may prevent formation of ice thereon. In one design shown in U.S. Pat. No. 4,246,963, elongated rounded surface hollow header bars traverse the cold air inlet of the heat exchanger to minimize ice formation thereon. However, these header bars do not impact stratification of the air flowing from the turbine outlet prior to the airflow entering the heat exchanger inlet face.
As can be seen, there is a need for an apparatus and method that improves the uniformity of a high velocity, sub-freezing fluid flowing from a turbine outlet, as the airflow contacts the inlet face of a heat exchanger. The need extends to preventing blockage of the heat exchanger face by ice and snow, to minimize stratification of the airflow and ice entering the heat exchanger, and to allowing the systems to operate at optimum efficiencies without the addition of systems, energy demands, and weight detrimental to overall performance.